1. Field of the Invention
The present invention relates to a control arrangement for demodulating an incoming radio frequency signal transmitted via a satellite, and more specifically to such an arrangement using Single Channel Per Carrier Demand Assigned Multiple Access (SCPC-DAMA) techniques.
2. Description of the Prior Art
It is known in the art that the SCPC-DAMA is an important variation of an FDMA (Frequency Division Multiple Access) technique. The terminology SCPC refers to the fact that each carrier is modulated by a bit stream representing a single user's voice channel. The SCPC is suited for a satellite communications system which comprises a number of small scale earth stations.
The SCPC carriers in the demand assignment (viz., DAMA) are voice-activated such that carrier power is turned on only during time intervals when the voice envelope exceeds a threshold level. More specifically, in the demand assignment, all channels are previously pooled so that any pair of earth stations may set up satellite circuits using idle channels in a burst mode when a traffic demand arises. When calls are terminated, the channels are released back to the pool, and the traffic demand waiting earth stations communicates with each other through continuous signals instead of bursts.
To accommodate the voice activation of the carrier, each PSK (Phase Shift Keying) demodulator (for example) provided in a receive terminal must rapidly acquire the beginning of each speech segment (viz., burst). If this is not done, an initial portion of the burst will be lost.
A prior art demodulator, which is used in the SCPC in the demand assignment mode, will be discussed with reference to FIG. 1.
The FIG. 1 arrangement comprises a down-converter 10 interconnected with a demodulating section 12. The down-converter 10 includes a voltage controlled attenuator 14 to which an incoming radio frequency (RF) signal is applied via an input terminal 16, a mixer 18 operatively coupled to a local oscillator 20, and an amplifier 22. On the other hand, the demodulating section 12 includes a fixed attenuator 24, a coherent demodulator 26, a carrier and clock recovery circuit 28, and a level detector 30.
The RF signal takes the form of a PSK (phase shift keying) modulated signal (for example), and is applied to the voltage controlled attenuator 14 via the terminal 16. The attenuator 14 reduces the amplitude of the RF signal under the control of an AGC (automatic gain control) signal 51 applied thereto from the level detector 30. The output of the attenuator 14 undergoes mixing at the next stage including the mixer 18 and the local oscillator 20. A modulated IF (intermediate frequency) signal is outputted from the down-converter 10 after being amplified at the amplifier 22.
The output of the down-converter 10 is then applied, via the fixed attenuator 24, to the coherent demodulator 26 and to the carrier and clock recovery circuit 28. The coherent demodulator 26 is supplied with a recovered carrier Rcr and a recovered clock Rclk, and synchronously demodulates the IF signal in a known manner and generates a demodulated baseband signal through an output terminal 32. The level detector 30 receives an analog baseband signal from a portion (not shown) forming part of the demodulator 26 and detects the level of the signal applied, and then supplies the attenuator 14 with the output thereof as the AGC signal 51.
The signal level of the incoming RF signal is undesirably lowered or changed due to the following reasons: transmission loss over a cable extending from the input terminal 16 to an outdoor unit (not shown), non-uniformity of gains of RF amplifiers which precede the converter 10, etc. In order to compensate for the undesirable level changes in the incoming RF signal, the variable attenuator 14 is provided with a relatively wide dynamic range (about 40 dB for example). However, this wide dynamic range adversely affects a quick response of the AGC loop shown in FIG. 1.
In the event that the FIG. 1 arrangement is applied to a satellite communications system in which the SCPC in the demand assignment mode is employed, the down-converter 10 is supplied with a continuous RF signal during a time period in the absence of voice activation, while being supplied with burst signals when voice-activated. As referred to in the above, to accommodate the voice activation of the carrier, the coherent demodulator 26 must acquire rapidly the beginning of each of the consecutively transmitted bursts.
As also previously mentioned, the voltage controlled attenuator 14 should have a relatively wide dynamic range (e.g. 40 dB) in order to compensate for the undesirable level deviations in the incoming RF signal, and hence the response of the AGC loop is rendered undesirably slow. Accordingly, the coherent demodulator 24 may fail to demodulate an initial portion of each of the bursts which are successively transmitted. It should be noted that the dynamic range of the fixed attenuator 24 is narrow (about 5 dB for example), so that the insertion of the attenuator 24 in the AGC loop does not adversely affect the AGC loop response. Further, it should be noted that the demodulation of a continuous RF signal is normally implemented with the arrangement shown in FIG. 1.
In addition to the above problems inherent in the prior art, the slow response in the AGC loop is unable to compensate for burst interval deviations which are caused by transmission power differences between the earth stations.